The present invention relates to mixed alkaline earth boroaluminate glasses and to aluminum nitride-compatible thick-film paste compositions containing mixed alkaline earth boroaluminate glass frits as a binder. The present invention also relates to a method for bonding a thick-film paste composition to an aluminum nitride substrate and to an aluminum nitride substrate having an electrically functional composition comprising a mixed alkaline earth boroaluminate glass frit binder bonded to at least a portion of a surface of the substrate.
Thick-film technology is an established, widely used method for efficiently producing high density, micro-electronic circuit patterns on various substrates, commonly referred to as hybrid circuits. The lead patterns and resistors in hybrid circuits are applied by sequentially printing thick-film conductor, resistor and/or dielectric paste compositions onto suitable ceramic substrates (e.g., alumina). Generally, thick-film paste compositions include an electrically functional filler material (i.e., a conductor, resistor or dielectric), a binder (e.g., a glass frit) that promotes bonding of the filler material to the substrate and a liquid vehicle, usually an organic compound or polymer, which serves as a dispersion medium for the inorganic components of the paste. In printing, paste compositions are forced through a stencil, mask or screen of the desired pattern and onto the substrate. After printing, the coated substrate is dried and fired to bond the printed patterns to the substrate. When combined with discrete add-on components (e.g., chip devices), a thick-film, hybrid circuit is created.
Substrates provide the mechanical base and electrical insulating material onto which thick-film hybrid circuits are printed. The majority of substrates used in fabricating thick-film hybrid circuits are produced from ceramic materials because of mechanical strength, electrical resistivity over broad temperature ranges, chemical inertness and thermal conductivity. Alumina, which provides a good combination of these various properties, is currently the most widely used thick-film substrate. However, circuit density has reached levels today where considerable heat must be dissipated, and the thermal conductivity of alumina and other conventional substrate materials is often inadequate to allow sufficient cooling for the circuit elements to function properly. Thus, identifying suitable alternative substrates having improved thermal conductivity is becoming increasingly important in order to satisfy the thermal management requirements in today's higher speed microelectronics packaging.
When heat transfer and dissipation are a problem, beryllia substrates have been employed. However, beryllia is somewhat weaker than alumina and is more expensive. Furthermore, beryllia is highly toxic as a powder or vapor and requires special handling when fired at high temperatures.
Due to the thermal properties it possesses, aluminum nitride offers great promise as a non-toxic alternative to beryllia as a substrate material for use in high-power applications. Aluminum nitride exhibits high thermal conductivity, ranging from about 130 to over 200 W/m.multidot.K, as compared to about 20 W/m.multidot.K for alumina. Aluminum nitride also possesses a relatively low coefficient of thermal expansion (CTE) between about 4 and about 4.5.times.10.sup.-6 /K, which is close to that of silicon and gallium arsenide. As a result, aluminum nitride is suitable for direct attachment of very-large-scale integration (VLSI) dies. The combination of high thermal conductivity and low CTE gives aluminum nitride good thermal shock resistance. Furthermore, aluminum nitride has a flexural strength exceeding that of alumina and beryllia, exhibits a low hardness which enables it to be machined easily and is stable at temperatures in excess of 900.degree. C. in an oxidizing environment and up to 1600.degree. C. in a reducing environment.
Despite the promise of aluminum nitride, application of thick-films on aluminum nitride or aluminum nitride-containing substrates is severely limited by the lack of compatible thick-film paste compositions which adhere sufficiently to such materials. In order to function as an effective thick-film paste composition, the glass frit binder component must remain chemically stable in atmospheric moisture for periods of years and be chemically compatible with the substrate. The proven alumina-compatible, thick-film paste compositions are not compatible with aluminum nitride during firing. Most glass frit binders originally developed for thick-film pastes printed on alumina substrates contain substantial amounts of lead oxide which is thermodynamically incompatible with aluminum nitride. Aluminum nitride is oxidized by lead oxide during firing, producing nitrogen gas which physically disrupts and blisters the film and thereby destroys patterning and reduces thermal conductivity. Furthermore, most of the common glass fluxes used to make glasses having low dilatometric softening temperatures, as is required for thick-film glass binders, contain additives (e.g., ZnO, P.sub.2 O.sub.5 and CdO) which are chemically incompatible with aluminum nitride during firing. Thus, a stable glass for use as a binder in aluminum nitride-compatible thick-film paste compositions is urgently needed.